Blender not saving rendered stills
If you render using F12 or the Render button, the image doesn't get saved automatically (I don't know why the deveolopers made that choice).

You can manually save the rendered image in the UV/Image editor Pressing F3 (or "save as image" on the menu).

To auto-save a rendered image in the format specified in the ouptut settings, you would need to press the Animation button (or Ctrl+F12).

Just remember to set the frame range so that it renders only one frame and not the default 250.